While the invention is applicable generally to macrofouling by macroinvertebrates of various types, it was developed in response to rapidly spreading infestations of zebra mussels, Dreissena polymorpha, and will be described with respect to its application to zebra mussels. The zebra mussel, which is native to Europe, was accidentally introduced into the United States Great Lakes in around 1985 or 1986. Since their introduction into the United States, zebra mussels have been found in the Hudson, Ohio, Susquehanna, Illinois, Mississippi and Tennessee Rivers and are expected to spread throughout the northern two-thirds of the United States. This area contains seventy percent (70%) of United States power plants.
The zebra mussel is of particular concern because of its fast rates of reproduction and growth, as well as its adaptability to changing environmental conditions, which allow it to flourish in water intakes and other related structures, thereby fouling them and rendering them ineffective or inoperative. More effective measures than those found in the prior art are therefore necessary to combat zebra mussel infestations.
Currently used methods of control fall generally into four categories: chemical; coatings; abrasive; thermal. Of these methods chemical controls are generally considered to be the most effective, both in performance and cost. Chemical methods include: continuous addition of chlorine or bromine; intermittent addition of bromine to kill veligers (the free-floating, planktonic, larval stage of the zebra mussel); various biocides; and chemical cleaners. The choice of one of the foregoing treatments depends on conditions and constraints of particular applications.
The coating of surfaces subject to zebra mussel infestation can be effective in controlling fouling. Various coating materials can be used in coating and may be used by itself or it may be used to increase the effectiveness of other methods with which it is used. Coatings, however, have several disadvantages: some materials may release elements into the water stream which are environmentally unsafe; coatings may be difficult to apply to equipment; and coatings generally require periodic inspection and reapplication.
Abrasive methods of macrofouling control can also be used. These methods include simple manual removal of mollusks, cleaning with high pressure water streams, and "sand blasting". The disadvantages of abrasive methods include: the need for protection of non-metallic surfaces such as expansion joints and valve seals; the extensive piping systems which are required for water jet cleaning; the labor-intensive nature of these methods; and the necessity of removing spent abrasive with methods such as "sand blasting".
Thermal treatments are also a well known method of controlling mollusk macrofouling. Thermal treatments include instantaneous and/or gradual increases in water temperature as well as direct application of steam to kill the mussels. Of these treatments, gradual increase of water temperature is the more commonly used method. This is generally effected by reversing the flow of water through a power plant so that heated water flows from the condenser to the water intake. This thermal backflushing has numerous disadvantages including: the necessity of a special plant design allowing reversal of the flow of water to increase intake water temperature; the expense of retrofitting plants which are not so designed; the difficulty of treating auxiliary systems (i.e. those other than the water intake systems); the increased wear on equipment due to the increased temperature of water at the intakes; the expense of shutting down plants which are not designed to reverse flow during operation; and the ability of zebra mussels to adapt rapidly to gradual changes in water temperature.
The disadvantages of prior art methods and apparati for thermal treatment of mollusk macrofouling are at least as well known as thermal treatment itself and have caused people in the field to dismiss the usefulness of this method of zebra mussel control. Numerous articles and authorities in the field mention thermal control in reviewing control options, but reject it as impracticable, cost ineffective or otherwise unworkable.
For example, Henk A. Jenner recognized in 1983 that "[t]o apply[ ] heat treatment[,] cooling water systems have to be designed specifically for this purpose." Henk A. Jenner, Control of mussel fouling in the Netherlands: experimental and existing methods, in SYMPOSIUM ON CONDENSER MACROFOULING CONTROL TECHNOLOGIES: THE STATE OF THE ART, 18-12 (Electric Power Research Institute, December 1983). The necessity of having a plant designed specifically to accommodate thermal treatment is echoed in numerous other references. Michael Khalanski, Testing of five methods for the control of zebra mussels in cooling circuits of power plants located on the Moselle River, in PROCEEDINGS: THIRD INTERNATIONAL ZEBRA MUSSEL CONFERENCE, 3-6 (Electric Power Research Institute, June 1993) ("Technical feasibility is the main problem, however, as it is extremely difficult to adapt heat treatment to systems not specifically designed for the purpose."); MACROFOULING CONSULTANTS, THE ZEBRA MUSSEL: ITS U.S. UTILITY IMPLICATIONS (1990) ("Use of this methodology may require expensive redesign and retrofitting in existing plants and could cause reduction in generating efficiency."). These technical difficulties are compounded by the fact that backflushing takes the cooling system out of service, thereby causing loss of service and adverse economic impact. ELECTRIC POWER RESEARCH INSTITUTE, ZEBRA MUSSEL MONITORING AND CONTROL GUIDE 7-68 (1992). Prior art methods of thermal treatment were thus considered infeasible or too costly to be implemented.
Writings in the field also describe the manner in which prior art thermal treatment of infestations can actually pose a threat to the water circulating systems which are being treated. Cooling water systems capable of thermal backflushing utilize their condensers to heat the cooling water and return it to the intake area. The condensers, however, are designed to operate with cool water flowing through them. The elevated temperature of the water flowing through the condensers can cause the thermal tolerances of vital equipment to be exceeded, possibly damaging the equipment. E. F. Neuhauser, et al., Thermal treatment to control zebra mussels at the Dunkirk Steam Station, in PROCEEDINGS: THIRD INTERNATIONAL ZEBRA MUSSEL CONFERENCE, 4-81 (Electric Power Research Institute, June 1993); Joseph C. Petrille and Michael W. Werner, A combined treatment approach using a non-oxidizing molluscicide and heat to control zebra mussels, in PROCEEDINGS: THIRD INTERNATIONAL ZEBRA MUSSEL CONFERENCE, 3-206 (Electric Power Research Institute, June 1993). Therefore, some systems which are capable of thermal treatment by backflushing water through the condensers cannot be safely treated by this prior art method. Id.